The present invention relates to a digital data transmission system which can be applied to such communication systems as satellite, ground-based network, mobile and cable communications, and more particularly, to a multi-level superposed amplitude-modulated baseband signal processor which receives a non-return-to-zero (NRZ) data stream and generates an output having a narrow frequency bandwidth and small side lobes even in a non-linear channel.
As compared with existing analog communication methods, digital communication methods have high reliability and enable the transmission of high-quality information. Therefore, digital communication methods are forming the mainstream in modern information communication, and the use thereof has gradually been increasing. The carrier wave parameters, for example, phase and amplitude, are modulated and transmitted in accordance with the information data to be transmitted. Then, the transmitted parameters are demodulated in the receiving portion, to thereby restore the data.
In the digital modulator which changes the digital data into a form transmittable via the proposed transmission channel, the modulator's performance is evaluated by the characteristics of bandwidth efficiency and electrical power efficiency.
The need for bandwidth-efficient digital modulation methods are increasing so that larger quantities of information may be processed in the restricted frequency resource environment, wherein adjacent carriers interfere with each other in densely used transmission channels.
Accordingly, when pure digital data is transmitted without any change, the bandwidth is wide. Therefore, in general, the input data is modulated and transmitted after limiting the bandwidth through filtering or other means.
In addition, good bandwidth efficiency can be obtained by gathering and transmitting a number of input data bits at one time in a single symbol unit selected among the symbols of a modulation signal. That is, a single symbol is formed by the input data being divided into bit groups consisting of k bits. When the data consisting of k bits is modulated so that the amplitude or degree of phase change is in proportion to 2.sup.k, which is the number of cases which k-bit data can express, a large amount of data can be transmitted even without extending the bandwidth on the power density spectrum. The reason for this is that the bandwidth is proportional to the symbol transmission speed, i.e., the reciprocal of the data symbol cycle, and remains unaffected by changes in amplitude or phase. This is called multi-level modulation.
The present invention aims to obtain a bandwidth-efficient modulation signal in a digital transmission system, and more particularly, to provide the function of generating a filtered and multi-level modulated baseband signal.
As a widely used modulation scheme for digital data transmission systems, the phase-shift-keying (PSK) and pulse amplitude modulation (PAM) methods respectively modulate the phase or amplitude of a carrier wave in accordance with the input data of binary code. If a single input data of binary code is modulated by being corresponded to a single phase, this is called binary phase-shift-keying (BPSK).
Further, a modulation method wherein two signals by the BPSK modulation are disposed to be divided into an in-phase and a quadrature-phase on the signal space, is called quadrature phase-shift-keying (QPSK). At this time, the input data corresponding to a phase of the single carrier wave is two-bit data.
Accordingly, when the same quantity of the data is transmitted, the bandwidth on the power density spectrum occupied by the QPSK signal is half that of the BPSK signal. Expressed another way, the QPSK method can transmit twice the information of the BPSK method, in the same bandwidth. Thus, a bandwidth-efficient modulation method, for example, 8-PSK or 16-PSK, is introduced.
In PAM, a symbol point for representing the multi-level amplitude of input symbols as described above, is located on the signal space. Then, multi-level quadrature amplitude modulation (QAM) signal is obtained by corresponding the symbol point to the multiple-bit input data.
As described above, the information transmitted by a single symbol in a single signal space increases as the degree of the multi-level becomes higher, which results in a more bandwidth-efficient modulation method.
Meanwhile, when the data input from the modulation methods described above is modulated and transmitted, the modulation signal occupies a very wide bandwidth. Therefore, before transmission, the bandwidth of the modulation signal is limited to maintain high performance.
Here, a method which forms and then transmits a bandwidth-limited signal using a raised-cosine filter which satisfies the Nyquist theory, is generally used. However, when a high-power amplifier is operated in a non-linear region, i.e., in saturation, for more power-efficient transmission, the side lobes are spread, which, as widely known, results in serious adjacent channel interference.
In addition, jitter, i.e., a time difference generated when the signal passes the zero level, increases as the bandwidth is limited, thereby causing difficulties in restoring the timing of the symbol in a demodulator.
Power and bandwidth-efficient digital signal transmission systems for preventing the above phenomenon are disclosed in U.S. Pat. No. 4,339,724 by Dr. K. Feher and U.S. Pat. No. 4,644,565 by Dr. J. S. Seo.
Signals generated by the above patents produce two pulse shapes which correspond to a single bit of the input data stream, which is a NRZ-shaped binary code. The signals of the produced pulse shapes, that is, a double-interval raised-cosine pulse and a common raised-cosine pulse, are superposed according to the ratio of the superposition (signal "A.revreaction. of FIG. 1) and then output via an output terminal. As a result, a superposed modulation baseband signal is generated, which minimizes amplitude fluctuation.
Accordingly, only slight re-growth of the side lobes occur and the possibility of error is low even when the superposed modulation signal where the superposed modulation baseband signal is used, that is, the modulated carrier obtained using the above superposed modulation baseband signal is amplified via a non-linear amplifier and transmitted via a communication channel. Therefore, a bandwidth-efficient and power-efficient modulation signal can be obtained while the inter-symbol interference (ISI) between the jitter and the code of the transmitting terminal does not occur.
Especially, in the invention by Dr. J. S. Seo, the main lobe bandwidth and the side lobe amplitude in power density spectrum can be controlled by controlling the superposition (A), which means that a suitably controlled bandwidth for a digital transmission system is possible. Also, when the superposition (A) equals one, the modulated output signal is identical to that of the invention by Dr. K. Feher.
The technical field and the characteristic of the above two patents are similar. Therefore, the signals generated by the above patents are called "superposed amplitude modulated baseband signals."
Different than the conventional filter which comprises a resistor, inductor, capacitor and an operational amplifier due to the characteristics of the superposed amplitude-modulated baseband signal, the devices of the inventions of the two patents for generating the superposed amplitude-modulated baseband signal use a non-linear method for obtaining the filtered and modulated signal. That is, devices of the inventions of the two patents generate a multitude of bandwidth-limited pulse waveforms corresponding to an output signal of the superposed amplitude-modulated baseband signal. Then, the pulse waveform selected among the pulse waveforms generated according to the pattern of the input data is output, to obtain a filtered and modulated signal.
Also, the inventions of the above patents generate a modulated signal from a single stream of input data, and are not multi-level amplitude modulation methods.
The impulse response characteristic y(t) is theoretically required to obtain the multi-level superposed amplitude modulation baseband signal, as follows. ##EQU1##
Here, a.sub.n is the amplitude of the multi-level signal space expressed by the input data to a point in time n, and corresponds to one element of the group consisting of .+-.1, .+-.3, .+-.5, . . ., .+-.(.sqroot.M-1), when the superposition number is M. In addition, s(t) is a baseband signal impulse response to the basic superposed amplitude-modulated signal, which is expressed as follows. ##EQU2## where A is a superposed degree and T.sub.s is a symbol duration.
In order to obtain the baseband signal of the multi-level superposed amplitude modulation, the element that constitutes the baseband waveform when the superposed amplitude modulation signal is multi-level superposed amplitude-modulated is generated within the processor. Then, one selected from the pulse waveforms generated according to the input data form is output.
However, as the number of multi-levels increases, the required number of pulse waveforms also increases geometrically, and the process of selecting and outputting a pulse waveform among the pulse waveforms becomes more complicated, thereby increasing the complexity of the processor.
As another conventional method for obtaining the multi-level superposed amplitude modulation signal, there is a method wherein the impulse response of the multi-level amplitude-modulated baseband signal is sampled and a digital filter having the value of the sampled signal as its coefficient is used. This method also requires that, as the number of multi-levels increases, various digital filters be added in parallel in correlation to the multilevel increase. As a result, the constitution and complexity of the circuit is increased.
All of the conventional multi-level superposed amplitude-modulated baseband signal generating methods generate a superposed amplitude-modulated signal which has a predetermined number of multi-levels. Therefore, when the number of multi-levels is required to be changed for flexibility of transmission systems, some of the generated pulse waveforms have to be included or omitted according to the change.
To perform this task, since all of the elements of the circuits such as digital filters have to be changed according to the change in the number of multi-levels at the worst case, the conventional method excessively complicates changing the number of the multi-levels, and hence lacks flexibility.
As another method for changing the number of multi-levels in the conventional multi-level superposed amplitude baseband signal generating method, there is a method in which the pulse signals relevant to the required number of the multi-levels are provided in the processor first, and the pulse signal is selected and used according to the number of the required multi-levels. This method also entails a large and complicated circuit.